As already discussed, the mechanisms of multiple chemical sensitivity & environmental injury are only partly understood but usually start with one well-defined “toxic event” & this is often exposure to one toxic chemical, but, what comes as a surprise to many outsiders is that multiple chemical sensitivity can also be caused by a physical event or occurance, such as, for example, a car crash or a knock on the head.

Looking at the “toxic chemical” route first, this is often a single gross exposure or repeated exposures to one toxin which thereafter produces a true allergic reaction on repeated exposure to the same toxin. This is a true allergy otherwise known as “sensitization”.
This toxin (excitant) could be one chemical, for example, formaldehyde, or one material, for example, wood dust.
Other common triggers are listed above - and in our alphabetical trigger list - & the possibilities are endless – a new brand of cosmetics, for example, could be the culprit.
Secondly – looking at the physical route for the initialization of multiple chemical sensitivity – environmental injury, this could be almost any injurious physical event.
This seemingly strange phenomenon is a well documented & often-reported feature of multiple chemical sensitivity & in our experience alone, has been reported to us many times by sufferers; in fact, whether caused by the physical OR chemical route, multiple chemical sensitivity is often referred to as “environmental injury” – when definitely associated with exposure to chemical toxins, it is often referred to as “chemical injury”.
Broadly speaking however, the physical route of initialization of multiple chemical sensitivity is NOT well understood, although in the USA, in a study into the effects of the drug Neurontin, one part of the study specifically linked brain injury to the onset of multiple chemical sensitivity, concluding that the injury allowed easier sensitization to MCS excitants to occur than would otherwise have been the case.
Once sensitized, via a chemical OR physical route, the experience for the multiple chemical sensitivity sufferer is much the same – an excessive over-reaction to everyday pollutants.
If one exposure to a chemical toxin in our modern – day environment can bring about such dramatic changes in one individual, then why is not everyone in the modern world suffering from multiple chemical sensitivity, environmental illness, chemical injury, chemical sensitivity or multiple allergy? After all, and as we all now know, toxicants are all around us, in our homes, in our workplace, in the foods we buy & in the products we use and even in the fresh air out of doors.

Simply put, some people have a propensity to developing multiple chemical sensitivity & other allergy-related ailments because of their inherited genetic make-up.

There are two ways this can work.
Firstly, due to a single gene inherited problem (for example, lactase deficiency), and
Secondly, due to multi-gene inherited problems ( such as, for example, hay-fever, nettle rash, anaphylaxis).
Sufferers of both types are more likely than the average person to develop other problems such as food intolerance, cross-reactions and multiple chemical sensitivity – multiple allergy and both types of problem tend to run in families and these “pro-allergy” genes are not uncommon.
The tendency for the TRUE allergy multi-gene inherited problems to run in families is called ATOPY & the family members who suffer from their various allergic ailments are called ATOPIC ( as in atopic eczema = eczema which runs in families). Because many genes are involved in these problems (upto twenty), and because no two atopic individuals are identical, ( not even family members are identical), the faulty genes can be present in different combinations leading to differing allergic (atopic) problems for the different family members. ( compare myalgic encephalomyelitis).
So, for example, one may be asthmatic, one may suffer from hay fever, and one have nettle rash, - all of these are classic allergic ailments.
These faulty genes confer a predisposition to allergy, multiple chemical sensitivity, environmental illness & other problems, but whether these problems ever actually fully develop into severe ailments is also partly dependent on other, external, factors, as follows:

the avoidance of exposure to allergens in early life – especially during the first two years of childhood – is particularly important as this is when the young person is most susceptible to sensitization (allergy induction). It is even thought likely in some quarters that sensitization can occur in the womb before birth due to allergens absorbed by the mother. As already discussed, the “pro-allergy” genes are not uncommon, and , as our modern lifestyle becomes ever more conducive to the acquisition of multiple allergy, so an individual needs fewer & fewer of these faulty genes in order to actually develop multiple chemical sensitivity / chemical injury. This is why the avoidance of early exposure to allergens (even pre-natal exposure) is so important and especially so to acknowledged Atopic families or to those who have other family members who are known to suffer allergies, multiple chemical sensitivity, chemical injury or myalgic encephalomyelitis.

an overly clean environment. To the uninitiated, this may seem like a ludicrous proposition, - that a home environment could be too clean for a baby or toddler, but this is true, and here is why: - The acquisition of true allergy involves the intervention of what are known as T1 and T2 helper cells, but an overly clean environment can mean that the young child does not encounter dirt, microbes, bacteria, or other micro-organisms in the type or quantity that it would in nature and so the natural & harmless “conditioning” of the immune system does not take place as it should – it is not “tested” and so remains under-developed but with a more pronounced tendency to allergy ( as brought about by T2 helper cells). In this scenario, the term “helper cells” is rather a misnomer, as it is these cells that are causing (mediating) the allergic response. In a natural environment, the small amount of dirt that the child would invariably ingest throughout it’s childhood actually helps to develop a fully functioning & healthy (non-allergic) immune system.

early weaning (from breast milk). In order to avoid problems of allergy, and especially when due to Atopy, it is very important that the child is fed mother’s milk (exclusively) for at least the first four months of life. Cow’s milk, or other milks (or later, foods) are not the same as mother’s milk. If Atopic eczema is a problem for the baby – in spite of breast-feeding – consult your doctor; a hydrolysed “formula” or hydrolysate may be better for a baby in an Atopic family, as it could be picking up allergenic foods through the mother’s milk. These are problems caused by a True allergy, but are also highly relevant for those with multiple chemical sensitivity and their families due to their tendency for cross-reactions.

a “modern” environment & lifestyle. There is very little that we can do to fundamentally change the fact that we live in a modern world. The reasons for this factor being involved in multiple chemical sensitivity and environmental illness are not fully known, but it is thought that this may include such diverse topics as: vaccination, antibiotics, pesticides, herbicides, toxic chemicals in bought goods, built homes, food & drink, food additives, chlorine in tap water, indoor & outdoor air quality – no-one really knows the full extent of the effects of these factors, but what is known is that these are some of the hallmarks of a modern Westernized society and that basic indigenous peoples all over the world living a simple & natural lifestyle often have no words for asthma or allergy or hayfever, because these conditions & others like them quite simply do not exist. However, once these same peoples take up a Westernized lifestyle, with all that that entails, then it is not long before the modern scourges of allergy, myalgic encephalomyelitis, multiple chemical sensitivity & food intolerance etc, gradually start to appear. At present, this phenomenon is inexplicable. On the surface, it seems that simply spending too much time indoors is bad for our health!

use of antibiotics. The use, or over-use of antibiotics in the child’s early life does certainly SEEM to be a precursor to allergy or allergy-like diseases such as multiple chemical sensitivity, although the mechanism for this is not understood. It is particularly important that any non-essential use of antibiotics is avoided, at least for the first two years of the child’s life.
In the case of serious disease, the use of antibiotics may not be avoidable, but it is wise not to request them for every “cough & sniffle” that the child
develops & to let the child build up their own natural defences.

In the natural world, Multiple Chemical Sensitivity, myalgic encephalomyelitis, asthma & many other similar “modern” ailments are simply not known, so what is the prevalence of multiple chemical sensitivity in the current world at large?
Three studies into the proportions of population suffering multiple chemical sensitivity reported that between 16% and 33% are “unusually sensitive or allergic to chemicals” - with about 4% reporting that their illness was an everyday occurrence. Two study papers concluded that multiple chemical sensitivity is of “global concern” and that the US and most of Europe is affected - although the problem is insufficiently researched, so hard evidence is not easy to come by.
The most widely cited triggers are: -

pesticides

cigarette smoke

paint fumes

wood preservatives

mercury amalgam ( in dentistry)

office photocopier fumes

perfumes

formaldehyde (MDF, insulation, bodycare, etc)

isocyanate ( foam insulation etc)

epoxy

As already described, those having a pre-existing tendency to allergy, rhinitis, hayfever, eczema, asthma, etc, are also more likely than the average to develop multiple chemical sensitivity.
A sensitivity to Electromagnetic radiations (EMR) and electrical field radiations, (ELF), commonly known as
Electrical Hyper Sensitivity (EHS) &
Electromagnetic Hyper Sensitivity from pylons & mobile phone masts and to Geopathic Stress due to, for example, underground cables or underground running water, has also been linked to a likelihood to develop multiple chemical sensitivity, but this is not as yet proven.